The invention relates to a method for controlling the signal level in a digital receiver. The invention also relates to an apparatus for controlling the signal level in a digital receiver.
With various digital receivers, such as digital TV receivers, it is desirable that the digital input signal level be at all times optimal. Due to losses in transmission networks and other non-idealities, real signal levels received from a transmission network may vary a great deal. To level out such variations digital receivers typically are equipped with various signal level control devices which aim at keeping the digital signal level constant regardless of the signal level of the signal received from the transmission network. These systems are generally called automatic gain control (AGC) systems. For example, many demodulator circuit manufacturers are offering circuits with an AGC output that provides a voltage dependent of the signal level of the input signal.
FIG. 1 shows a prior-art arrangement 10 for realizing signal level control in a digital receiver, based on a circuit 14 with an AGC output. The arrangement 10 is based on the use of two successive control loops 11 and 12. A first control circuit 11 controls the signal level of a received analog RF signal and filters by means of a bandpass filter 18 the desired frequency band to be sent forward. A second control loop 12 controls the level of the filtered analog IF signal produced by the first control loop 11. For simplicity, FIG. 1 does not show components, such as mixer elements, which are essential to the operation of the receiver but inessential as regards the operation of the control circuits.
From the second control circuit 12 the analog signal is taken to an A/D converter 13 which converts the analog signal into a digital one. The digital signal thus produced is fed to an AGC circuit 14 which may be a demodulator, for example. The AGC circuit 14 estimates the level of the digital signal coming from the A/D converter 13 and produces an AGC signal 19 proportional to the signal level. Since variable gain amplifiers usually are controlled using an analog signal, the AGC signal has to be D/A-converted. This can be advantageously realized internally in the AGC circuit 14 so that the AGC circuit 14 directly produces an analog AGC signal 19 or, if the circuit 14 has no D/A converter, in a separate D/A converter circuit (not shown). The AGC signal 19 is taken direct to a variable-gain amplifier 16 in the second control circuit 12 and via a threshold circuit 17 to a variable-gain amplifier 15 in the first control circuit 11.
FIG. 2 is a graph illustrating how the level of the analog signal, which is produced by the second control circuit 12 in the arrangement 10 according to FIG. 1 and fed to an A/D converter 13, affects the second control signal (IF Gain) controlling an amplifier 16 in the second control circuit and on the first control signal (RF Gain) following the threshold circuit 17 and controlling an amplifier 15 in the first control circuit 11. Stepwise dependence of amplification factors on the value of the analog signal is the result of quantization noise caused by the limited conversion accuracy of the D/A converter in the AGC circuit 14 or separate D/A converter circuit. Quantization noise is caused by the fact that digital apparatus only have a limited number of bits at their disposal so that the smallest possible adjustment step that can be achieved with digital systems equals the operating range divided by the number of bit combinations available. For example, a one-volt control range in an 8-bit system (256 bit combinations) produces a smallest possible adjustment step of about 4 mV (1 V/256).
In the example depicted in FIG. 2 the system is specified such that when the level of the analog signal coming from the second control circuit 12 is at its minimum, both amplification factors are at their maximum. The behavior of the amplification factors may be realized in other ways, too. When signal levels start to rise in the example depicted in FIG. 2, the amplification factor (IF Gain) of only the second control circuit 12 is reduced at first. The reduction of the amplification factor (RF Gain) of the first control circuit 11 is begun only when the signal level exceeds a certain threshold level 21 defined by means of a threshold circuit 17. This arrangement produces a noise performance which is optimal for the system whole.
Since the values of the amplification factors may vary greatly in an arrangement such as the one described above, the control range also needs to be wide. As the D/A conversion is realized using a limited number of bits, it subsequently follows that the control system always includes quantization noise which degrades the quality of the signal available by preventing accurate signal level control.
A solution obvious to a person skilled in the art for the prior-art quantization noise problem described above is to increase the number of bits available. In the simplest case this would require more accurate circuits, i.e. circuits with more bits, to perform the D/A conversion. AGC circuits with more accurate D/A converters are considerably more expensive as well as more limited in variety than the most common circuits used currently. In the ideal case the user could design and build a circuit with the necessary characteristics but then the unit price of the circuit would be significantly higher.
Another disadvantage of the prior art described above is the system""s sensitivity to disturbances. As the AGC signal 19 is used for directly controlling the amplifier 16 in the second control circuit 12, it is possible that a small spurious signal may cause the digital control system to oscillate between two or more amplification values such that the oscillation continues even after the spurious signal has disappeared.
A solution obvious to a person skilled in the art for the disturbance-sensitivity of the arrangement according to the prior art would be to attempt to shield the apparatus as effectively as possible against electrical interference by placing it in a grounded metal casing, for example. Such protective systems add to the size and manufacturing costs of the apparatus but will not, however, provide adequate protection e.g. against spurious signals accompanying the received signal.
An object of this invention is to provide a new method and apparatus for controlling the signal level in a digital receiver, eliminating the above-mentioned disadvantages of the prior art. These objects are achieved by realizing coarse control of signal level by means of an analog control circuit and fine-controlling the resulting signal by means of a digital control circuit which is functionally separate from the analog control circuit.
More specifically, the present invention is a method for controlling signal level in a digital receiver, comprising at least the following steps: an analog signal is received from a transmission network or the like, the level of the received analog signal is controlled in an analog control circuit, whereafter the analog signal is converted to a digital signal in an A/D converter, wherein the level of the analog signal coming from the analog control circuit is controlled in a digital control circuit which is functionally substantially separate from the analog control circuit, and the amplification factor of the digital control circuit is adjusted on the basis of the level of the digital signal coming from the A/D converter.
The present invention is also an apparatus for controlling signal levels in a digital receiver, comprising at least an analog control circuit for controlling the level of an analog signal, wherein the apparatus includes a digital control circuit, which is functionally substantially separate from the analog control circuit, for controlling the level of the analog signal coming from the analog control circuit.
Compared to the prior art, an advantage of the invention is better interference tolerance as spurious signals of short duration will not bring the control system according to the invention into long-lasting oscillations possible in the arrangement according to the prior art.
Another advantage of the arrangement according to the invention, compared to the prior art, is lower quantization noise, which improves the quality of the digital signal available.